To decrease rolling resistance of a tire so as to enhance low fuel consumption performance, it has been proposed that silica instead of carbon black is blended as a rubber reinforcing agent into the tread rubber. However, silica has poor conductivity. Therefore, in the tire using the tread rubber highly blended with silica, static electricity may accumulate in a vehicle, thus causing electromagnetic interference, such as radio noise.
Hence, for example, a tread rubber (a) as shown in FIG. 7(A) has been proposed. The tread rubber (a) includes a base rubber portion (b) of conductive rubber highly blended with carbon, a cap rubber portion (c) of non-conductive rubber highly blended with silica, and a conductive terminal portion (d) of conductive rubber extending upward from the base rubber portion (b) through the cap rubber portion (c) to a tread ground contact surface. The tread rubber (a) can be formed by extrusion molding of rubber.
To suppress uneven wear that occurs between the conductive terminal portion (d) and the cap rubber portion (c), an exposed width (Wd) of the conductive terminal portion (d) is preferably decreased as much as possible. The exposed width (Wd) is, for example, not more than 5 mm. When the exposed width (Wd) is set to a small value in the extrusion molding, there is a probability that due to variations of a rubber flow in an extrusion head, the conductive terminal portion (d) is interrupted therein and fails to be in continuity. At this time, it is difficult to easily find the interruption of the conductive terminal portion (d), thus eventually requiring a continuity test of a total number of tires. This significantly lowers productivity.
The following patent document 1 has proposed to form the cap rubber portion (c) and the conductive terminal portion (d) by using what is called a strip winding method including spirally winding the rubber strip instead of the extrusion molding, as shown in FIG. 7(B). The strip winding method has an advantage that the interruption of the conductive terminal portion (d) can be easily detected as a break of the rubber strip in the middle of winding.
This structure, however, requires the use of rubber strips (e1, e2) having a large width beyond a thickness of the cap rubber portion (c), thus making it difficult to form the tread rubber in a desired cross-sectional shape. Moreover, the rubber strips (e1, e2) have a posture corresponding to a steep slope. Therefore, when the rubber strip (e2) for the conductive terminal is bonded to a side surface (slope) of the rubber strip (e1) wound previously, the rubber strip (e2) is susceptible to misregistration along the slope, thus making it difficult to maintain highly accurate formation of the tread rubber.
It is therefore desired to form the conductive terminal portion (d) by continuously windingly laminating the rubber strip (e2) for the conductive terminal radially outward from the base rubber portion (b) to the tread ground contact surface.
However, when the conductive terminal portion (d) having the exposed width (Wd) of not more than 5 mm in the winding laminate structure, the width of the rubber strip (e2) also becomes narrow, as small as not more than 5 mm, as shown in FIG. 8. Therefore, a laminate collapse may occur when laminating the rubber strip (e2), thus failing to stably form the conductive terminal portion (d).